I finally got my system going last night (replaced the circ pump motor), but one thing has me scratching my head. The glycol level somehow rose to the top of the clear tube. I drained it so that it was back at about halfway up, but it again filled up to the top. I just drained it again today (1.5 gallons worth) to get it back to half way, and it's already almost to the top. What would cause that?